1. Field of the Invention
This invention relates to a method of and system for controlling ignition timing in an internal combustion engine, and particularly to improvements in a method of and a system for controlling ignition timing in an internal combustion engine, suitable for use in an engine of a motor vehicle provided with an intake-air-pressure-sensing type of an electronic ignition timing control device, and in which a throttle valve and an intake air control valve independent of the throttle are used, and ignition timing is controlled in accordance with the engine operating conditions.
2. Description of the Prior Art
As one of the methods of controlling intake air fed to combustion chambers of an internal combustion engine, such as a motor vehicle engine, a throttle valve is used for controlling the total flowrate of intake air fed to the combustion chambers, and an intake air control valve is controlled in accordance with the engine operating conditions, for controlling part of the intake air (See, e.g., U.S. Pat. No. 4,246,874 and U.S. Pat. No. 4,271,795). The above-described internal combustion engine provided with the throttle valve and the intake air control valve independent of the throttle valve features that, in accordance with the engine operating conditions, during low speed rotation of the engine, for which a comparatively low intake air is required for example, the intake air control valve is closed to increase the velocity of flow of the intake air, so that the combustion in the combustion chambers can be carried out smoothly, whereas, during high speed rotation of the engine, the aforesaid intake air control valve is opened to quickly feed a required intake air flowrate to the combustion chambers, whereby the high speed rotation is carried out smoothly, so that the low speed operating performance and the high speed operating performance of the engine can be compatible.
Additionally, as one of the methods of controlling ignition timing of an internal combustion engine such as a motor vehicle engine, an electronic ignition timing control system has been used. According to this method, the ignition timing is electronically controlled in accordance with the engine operating conditions, so that an air-fuel mixture can be ignited at a predetermined ignition timing.
In consequence, in providing the aforesaid electronic ignition timing control system on the engine having the aforesaid intake air control valve, the ignition timing has heretofore been determined in accordance with engine load detected from intake pressure of the engine, for example, in the same manner as in the engine having only the ordinary throttle valve. However, the internal combustion engine having such intake air control valve has operated at a disadvantage because, even if the apparent engine load, i.e. intake air pressure, is the same, there are cases in which required ignition timings are varied depending on whether the intake air control valve is opened as indicated by a broken line A or the intake air control valve is closed as indicated by a one-dot chain line B in FIG. 1. Consequently, as against the above, if a single ignition timing is set as indicated by a solid line as C in the past, then the ignition timing satisfying the requirement cannot be obtained. This is particularly true of the intake-air-pressure sensing type of an electronic ignition timing control system for detecting the engine load from the intake air pressure.
In order to obviate the above-described disadvantage, it is conceivable to change to set ignition timing in accordance with the on-off conditions of the intake air control valve (See e.g. Japanese Laid-Open No. 48776/83). However, if the set ignition timing is changed in response to a change-over command for the intake air control valve, then, as shown in FIG. 2, there is a possibility of a discrepancy in timing, as shown in a comparative example as indicated by broken line E, because a certain period of time Td is required until the opening degree of the intake air control valve is actually varied, the conditions of combustion are changed and the required ignition timing indicated by a solid line D is varied, after the mechanical operation of the intake air control valve has been started. In consequence, in the case of the comparative example shown in FIG. 2, for example, the ignition timing is excessively advanced to cause knocking because the required ignition timing is set at the time when the intake air control valve is opened regardless that the conditions of combustion are still varying.